Anti-scatter grid with mechanical resistance

ABSTRACT

An anti-scatter grid for radiology imaging having an anti-scatter layer with a plurality of metallized partitions that enable X-rays emitted from a source located above the grid to pass and absorbing X-rays not derived directly from this source. The grid has at least one plate of expanded polymer material fixed on one surface of anti-scatter layer. The grid may be positioned with a frame.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of a priority under 35 SC119(a)-(d) to French Patent Application No. 03 06139 filed May 22, 2003,the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

[0002] The invention concerns anti-scatter grids as used in radiologyimaging and particularly in X-ray imaging.

[0003] A radiology imaging apparatus conventionally comprises s sourceof radiation, such as an X-ray source, and a means for forming theimage, such as an image receptor, between which the object to be imagedis positioned. The beam of radiation emitted by the source passesthrough the object before reaching the receptor. It is partly absorbedby the inner structure of the object so that the intensity of the beamreceived by the receptor is attenuated. The global attenuation of thebeam after passing through the object is directly related to absorptiondistribution within the object.

[0004] The image receptor comprises an optoelectronic detector orintensifying screen-film couple, sensitive to radiation intensity.Consequently, the image generated by the receptor corresponds inprinciple to the distribution of global ray attenuation subsequent topassing through inner structures of the object.

[0005] Part of the radiation emitted by the source is absorbed by theinner structure of the object, the other part is either transmitted(primary or direct radiation) or scattered (secondary or scatterradiation). The presence of scatter radiation leads to degradation ofcontrast in the image obtained and a reduced signal to noise ratio. Thisis of particular hindrance, in particular if it is desired to visualizedetails of the object.

[0006] One solution to this problem comprises inserting an“anti-scatter” grid between the object to be X-rayed and the imagereceptor. These grids are usually formed of a series of parallel stripsor partitions of X-ray absorbing material. In so-called “focalized”grids (according to the terminology laid down by standard IEC 60627 on“X-ray imaging diagnostic equipment—Characteristics of anti-scattergrids for general use and mammography screening”) all the planes of thestrips or partitions are oriented along planes passing through the focalpoint of radiation emitted by the source. Therefore, these grids allowdirect radiation to pass and absorb scatter radiation. Focalizedanti-scatter grids have contributed towards a considerable improvementin the contrast of images obtained.

[0007] In order to obtain good quality images it is desirable to providegrids having the finest possible structure so as not to disturb directradiation. It is also desirable to control the orientation of theabsorbing strips or partitions with precision. The precision with whichthe strips or partitions are orientated evidently depends upon themanufacturing technique used to produce the grid. However, it is foundthat during use of the grid it may undergo deformation, whichsubstantially modifies strip orientation. The consequence is impairedprecision of strip or partition orientation. This impairment is greaterthe narrower the thickness of the grid and its propensity to deform.

[0008] This problem is particularly raised in imaging devices with anoverhanging grid, i.e., fixed on one side only. In this case it mayundergo substantial bending stresses.

[0009] To overcome these disadvantages, grids have been proposed havingan aluminium frame, the frame giving rigidity to the assembly. Inaddition, these grids are coated on each of their surfaces with platesin a composite carbon and resin material having a thickness of between0.2 and 0.4 mm.

BRIEF DESCRIPTION OF THE INVENTION

[0010] An embodiment of the invention is directed to an anti-scattergrid comprising an anti-scatter layer having a plurality of metallizedpartitions, these partitions allowing radiation that are emitted from asource located above the grid to pass and absorbing those radiationwhich do not derive directly from this source and at least one plate inan expanded polymer material fixed to one surface of the anti-scatterlayer.

[0011] An embodiment of the invention also directed to a method forfabricating an anti-scatter grid comprising:

[0012] forming an anti-scatter layer having a plurality of metallizedpartitions, these partitions enabling radiation to pass emitted by asource located over the grid and absorbing those radiation which do notderive directly from this source; and

[0013] fixing at least one plate of expanded polymer material on onesurface of the anti-scatter layer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The invention and embodiments thereof will be better understoodfrom the following description that is solely illustrative andnon-restrictive and is to be read with reference to the appended figuresin which:

[0015]FIG. 1 is a schematic view of an anti-scatter layer of a focalizedgrid;

[0016]FIG. 2 is a schematic view of the layers forming an anti-scattergrid according to an embodiment of the invention;

[0017]FIG. 3 is a schematic view of a frame intended to hold thegrid-forming layers;

[0018]FIG. 4 is a schematic view of the positioning of two side parts ofthe frame;

[0019]FIG. 5 is a schematic view of the positioning of two other partsfinalizing this frame; and

[0020]FIG. 6 is a schematic view of a crosspiece intended to hold inplace the grid-forming layers.

DETAILED DESCRIPTION OF THE INVENTION

[0021] In FIG. 1 anti-scatter layer 10 is formed of a planar substrate12 of a polymer material, approximately 1 to 3 mm thick, comprisingpartitions defining cells 14. As shown in FIG. 1, the thickness cantypically be 1.7 mm. The inner walls of cells 14 are coated with anabsorbing metal layer 16. Anti-scatter layer 10 is focalized, whichmeans that the cell walls are oriented along planes passing through thefocal point of radiation emitted by a source of radiation.

[0022] As a result of the anti-scatter layer 10, part of the directradiation emitted by an X-ray source passes through the grid viasubstrate 12 while another part passes through the layer via cells 14.On account of the low density of the polymer forming substrate 12, theradiation passing through it is scarcely attenuated.

[0023] The inner walls of cells 14 coated with a metal layer 16 absorbscatter radiation arriving at anti-scatter layer 10 at too great anangle relative to the direction of focalization of one of cells 14.

[0024] In FIG. 2, two plates 20 and 22 of expanded polymer material arearranged on each surface of anti-scatter layer 10. The polymer materialforming the plate should have sufficient rigidity to prevent griddeformation and sufficient homogeneity so as not to disturb the X-rayimage through artefacts. Expanded polymer materials have the advantageof scarcely attenuating X-rays on account of their low surface density.The plate of expanded material also plays a protective role for theanti-scatter layer of the grid.

[0025] Both plates 20 and 22 may be formed of a hard polymethacrylimide(PMI) foam. This type of foam is manufactured, for example, by RÖHM GmbHunder the trademark ROHACELL® or an expanded polyetherimide (this typeof material is supplied for example by ALCAN AIREX AG under thetrademark AIREX®). The plate is formed in a material having a density ofbetween 20 and 70 kg/m³. ROHACELL® is available in this density range.In particular a density in the order of 30 kg/m³ is available. Theplates may have a thickness between 2 and 6 mm and the two plates mayhave the same thickness.

[0026] Plates 20 and 22, respectively positioned on the surface ofanti-scatter layer 10 are intended to be exposed to the rays emitted bythe X-ray source, and on the surface of anti-scatter layer 10 located onthe image detector side, can be identical. The thickness of the platesis on the order of 3 mm with an approximate density of 30 kg/m³. Asshown in FIG. 2, there are two plates 20, 22 with a range of thicknessbetween 2 and 4 mm.

[0027] Assembly of plates 20 and 22 of polymethacrylimide is made bybonding. The adhesive is preferably deposited on plates 20 and 22 andthese plates are then superimposed on anti-scatter layer 10.The adhesivemay be distributed so that it only contacts a peripheral area ofanti-scatter layer 10 which does not form an active part of the layer.Therefore the adhesive does not disturb radiation transmission throughlayer 10 and plates 20 and 22.

[0028] Alternately, the adhesive may be placed so that it contacts theentire surfaces of anti-scatter layer 10 which improves the mechanicalresistance of the assembly. In this case, an aerosol adhesive ispreferred to provide a fine, homogeneous layer of adhesive. This bondingtechnique avoids filling the cells of the anti-scatter layer.

[0029] It is also possible to use a film adhesive. This type of adhesiveis in the form of a film with or without a backing that can be depositeddirectly on a surface of each of plates 20 or 22 so that they can beassembled with anti-scatter layer 10. Adhesive films have the advantageof providing a thin, homogeneous layer of constant thickness andtherefore of obtaining constant radiation transmission over the entireassembly surface.

[0030]FIGS. 3 and 4 shows a frame 30 intended to be positioned aroundthe assembly formed by the anti-scatter grid. The purpose of frame 10 isto rigidify and to protect the assembly.

[0031] In FIG. 4, positioning of the frame comprises a first step inwhich a crosspiece 38 is positioned on one of the longitudinal sides ofthe assembly formed by the superimposition of plates 20, 22 andanti-scatter layer 10. The second step in frame positioning comprisesplacing two U-shaped 32 and 34 sections made of carbon compositematerial on the two opposite transverse sides of the assembly. TheU-shaped sections encase the assembly and crosspiece 38. As shown inFIG. 4, the thickness of the U-shaped portions of the sections 32 and 34may be approximately 1.0 mm. As shown in FIG. 5, the legs of theU-shaped sections 32 and 34 may be approximately 5.0 to 10. mm.

[0032]FIG. 5 shows a third step comprising depositing a fine layer 36(thickness on the order of 0.3 to 0.5 mm) of carbon composite materialon the remaining longitudinal side of the assembly to finalize frame 30.

[0033] The anti-scatter grid obtained (FIG. 5) is particularly adaptedfor mammography screening applications. The longitudinal side coatedwith fine layer 36 is the side against which the patient leans, and thelongitudinal side along which crosspiece 38 extends is the side on whichthe anti-scatter grid is held in place. With fine layer 36, X-rayspassing close to the patients' ribcage are not hindered so as to obtainthe most extensive mammography view possible. Crosspiece 38 is intendedto fix the anti-scatter grid for a Potter-Bucky device. Crosspiece 38limits vibrations of the anti-scatter grid should it be placed inmovement.

[0034] The anti-scatter grid may also comprise one or more protectionlayers covering one or optionally both plates 20 and 22 ofpolymethacrylimide. The protection layer may be formed of a polymermaterial for example, a composite material containing carbon fibers, alacquer or varnish. The protection layer is intended to protect theexpanded polymethacrylimide plate against humidity and impact. Theattenuation of X-rays by the protective layer should be the leastpossible. The protective layer is made of a polymer material for examplehaving a thickness in the order of 0.1 mm that provides an acceptableattenuation of X-rays in the order of 1%.

[0035] The protection layer can be a polymer material, preferably apolyester (supplied for example by DUPONT DE NEMOURS under trademarkMYLAR®) in polycarbonate (available from RÖHM GmbH for example undertrademark EUROPLEX®), or in polymethylmethacrylate PMM (supplied forexample by RÖHM GmbH under trademark PLEXIGLASS®).

[0036] The protection layer is preferably deposited on a surface ofplate 22 oriented in an opposite direction to the X-ray source (i.e.,towards the detector). The protective layer protects the grid againstpossible impacts during handling operations. However, plate 20 orientedtowards the source may also be given a protection layer.

[0037] In one variant of embodiment of the invention, the assembly maybe held in place by a crosspiece and not a frame.

[0038]FIG. 6 shows a crosspiece 38 intended to be positioned on one ofthe longitudinal sides of the assembly. Crosspiece 38 has a straightgenerally U-shaped section. The assembly, comprising the two plates 20and 22 in expanded polymer material and anti-scatter layer 10, isinserted between the two sides of the U. Crosspiece 38 is intended torigidify and to protect the edge of the assembly. Crosspiece 38 is alsoused to fix the assembly to a Potter-Bucky. Fixations may be providedfor this purpose on crosspiece 38. The grid so fabricated is lighterthan the grid in FIG. 5.

[0039] The plate of expanded material can rigidify the grid and maintainthe anti-scatter layer in its initial form. Expanded materials offer ahigh bending strength-to-weight ratio. In addition, these materials havelow surface density, which means they make practically no contributiontowards grid deformation.

[0040] One skilled in the art may make or propose various modificationsto the structure and/or way and/or function and/or result and/or stepsof the disclosed embodiments and equivalents thereof without departingfrom the scope and extant of the invention.

What is claimed is:
 1. An anti-scatter grid comprising; an anti-scatterlayer having a plurality of partitions; and at least one plate ofexpanded polymer material fixed to one surface of the anti-scatter grid.2. The grid according to claim 1 wherein the plate is inpolymethacrylimide or polyetherimide.
 3. The grid according to claim 1wherein the plate is formed of a material having a density of between 20and 70 kg/m³.
 4. The grid according to claim 2 wherein the plate isformed of a material having a density of between 20 and 70 kg/m³.
 5. Thegrid according to claim 1 wherein the plate has a thickness of between 2and 6 mm.
 6. The grid according to claim 2 wherein the plate has athickness of between 2 and 6 mm.
 7. The grid according to claim 3wherein the plate has a thickness of between 2 and 6 mm.
 8. The gridaccording to claim 4 wherein the plate has a thickness of between 2 and6 mm.
 9. The grid according to claim 1 wherein the plate is bonded tothe anti-scatter layer.
 10. The grid according to claim 2 wherein theplate is bonded to the anti-scatter layer.
 11. The grid according toclaim 3 wherein the plate is bonded to the anti-scatter layer.
 12. Thegrid according to claim 4 wherein the plate is bonded to theanti-scatter layer.
 13. The grid according to claim 5 wherein the plateis bonded to the anti-scatter layer.
 14. The grid according to claim 6wherein the plate is bonded to the anti-scatter layer.
 15. The gridaccording to claim 7 wherein the plate is bonded to the anti-scatterlayer.
 16. The grid according to claim 8 wherein the plate is bonded tothe anti-scatter layer.
 17. The grid according to claim 9 the bonding isan adhesive arranged on a peripheral area of the anti-scatter layer. 18.The grid according to claim 9 the bonding is an adhesive that forms athin film extending over the entire surface of the anti-scatter layer.19. The grid according to claim 18 the adhesive is an aerosol adhesivesprayed to form a film.
 20. The grid according to claim 18 the adhesivein film form.
 21. The grid according to claim 1 comprising: two platesof expanded polymer material, arranged on each of the surfaces ofanti-scatter layer.
 22. The grid according to claim 21 wherein the twoplates are of the same thickness.
 23. The grid according to claim 1comprising: a protection layer for one of plates.
 24. The grid accordingto claim 21 comprising: a protection layer for one of plates. 25 Thegrid according to claim 23 wherein the protection layer is a polymermaterial formed of a composite material containing carbon fibers, alacquer or a varnish.
 26. The grid according to claim 23 wherein theprotection layer has a thickness in the order of 0.1 mm.
 27. The gridaccording to claim 24 wherein the protection layer has a thickness inthe order of 0.1 mm.
 28. The grid according to claim 23 wherein theprotection layer is arranged on a surface of the plate oriented in adirection opposite to a means for providing a source of radiation. 29The grid according to claim 24 wherein the protection layer is arrangedon a surface of the plate oriented in a direction opposite to a meansfor providing a source of radiation.
 30. The grid according to claim 1wherein the partitions form a plurality of focalized cells.
 31. The gridaccording to claim 21 wherein the partitions form a plurality offocalized cells.
 32. The grid according to claim 1 wherein thepartitions a plurality of cells, inner walls of the cells being coatedwith a layer that absorbs radiation.
 33. The grid according to claim 21wherein the partitions a plurality of cells, inner walls of the cellsbeing coated with a layer that absorbs radiation.
 34. The grid accordingto claim 1 wherein the grid is positioned within means for protectingthe grid.
 35. The grid according to claim 21 wherein the grid ispositioned within means for protecting the grid. 36 A method forfabricating an anti-scatter grid comprising: forming an anti-scatterlayer having a plurality of partitions; and fixing at least one plate ofexpanded polymer material to one surface of anti-scatter layer.
 37. Themethod according to claim 36 wherein the plate is bonded to theanti-scatter layer.
 38. The method according to claim 36 comprising:forming a protection layer for one of plates
 39. The method according toclaim 37 comprising: forming a protection layer for one of plates 40.The method according claim 38 wherein the protection layer is arrangedon a surface of the plate oriented in a direction opposite to a meansfor providing a source of radiation.
 41. The method according claim 39wherein the protection layer is arranged on a surface of the plateoriented in a direction opposite to a means for providing a source ofradiation.
 42. The method to claim 36 comprising: positioning the gridwith means for protecting the grid.
 43. An anti-scatter grid comprising;an anti-scatter layer having a plurality of partitions; at least oneplate of expanded polymer material fixed to one surface of theanti-scatter grid; a cross-piece positioned on one side of an assemblyformed by the layer and the plate; respective U-shaped sectionspositioned on two opposite sides of the assembly; and a layer on anotherside of the assembly wherein the cross-piece, the sections and the layerform a frame in which the grid is positioned.
 44. An anti-scatter gridcomprising; an anti-scatter layer having a plurality of partitions; atleast one plate of expanded polymer material fixed to one surface of theanti-scatter grid; and a cross-piece positioned on one side of anassembly formed by the layer and the plate.
 45. An anti-scatter gridcomprising; an anti-scatter layer having a plurality of partitions; atleast one plate of expanded polymer material fixed to one surface of theanti-scatter grid; and means for protecting an assembly formed by thelayer and the plate.